The high cost of energy has led industry to extract usable heat from high temperature waste streams whenever practical. In practice, a heated waste stream passes over conventional crossflow heat exchanger tubes through which uncontaminated ambient air is flown. The uncontaminated ambient air is heated by the waste stream and then used in many applications, including facility heat and process heat such as wood chip drying and combustion preheating.
Initially, heat extraction from high temperature waste streams employed metal-tubed heat exchangers. However, three particular problems with metal tubes were immediately observed. First, the temperature limit of the metals was exceeded by the operating temperature of the heat exchanger. Second, the waste streams were frequently abrasive and/or corrosive and so posed a danger to the physical integrity of the metallic tubes. Third, the metallic tubes were typically positioned by welding them to the receiving walls of the heat exchanger, thereby hindering their ease of individual replacement.
In response to these concerns, high temperature-, abrasion- and corrosion-resistant ceramic tubes were developed to replace the metal tubes. However, because ceramics can not be welded to receiving walls, a potential for air leaks from the gap between the ceramic tube and the receiving wall was realized.
U.S. Pat. No. 4,632,181 ("the Graham patent") discloses a ceramic heat exchanger having a number of features which purport to solve these problems. See FIG. 1. A first feature is a bell-shaped threaded insert which screws into the receiving wall and exerts axial pressure upon the end of each ceramic tube, thereby stabilizing and providing easy access to the tube. A second feature of the Graham heat exchanger is a gasket placed between the ceramic tube and the insert to provide a sealing engagement at the insert-tube interface. As noted in the Graham patent, when the temperature in the heat exchanger rises, the tube expands and compresses the gasket against the insert, thereby forming a very good seal and reducing leakage to a minimum. See the Graham patent at column 3, lines 65-68. However, since the gasket provides an axial connection between the gasket and the insert/tube combination, the axial expansion of the tube also results in an outward pressure against the wall. At extreme temperatures or with longer tubes, this pressure forces the wall to bow outward, thereby creating stability problems and/or cracks resulting in air stream leakage. In addition, this pressure results in high compressive forces on the ceramic tube itself which could lead to stress-induced tube failure.
Accordingly, it is an object of the present invention to provide a ceramic heat exchanger in which axial expansion of its tubes does not result in undue pressure upon its walls or the ceramic tube, thereby maintaining a stable configuration.